1. Field of the Invention
The present invention relates to a process for removing unpolymerized gaseous monomers from solid olefin polymers; and more particularly and in a preferred embodiment, to a process for removing unpolymerized gaseous hydrocarbon monomers from granular, low pressure-polymerized, low density ethylene-hydrocarbon polymers.
2. Description of the Prior Art
Low density ethylene polymers (i.e., ethylene polymers having a density of about 0.94 g/cc and lower) have in the past been made commercially by a high pressure (i.e., at pressures of 15,000 psi and higher) homopolymerization of ethylene in stirred and elongated tubular reactors in the absence of solvents using free radical initiators. Recently, low pressure processes for preparing low density ethylene polymers have been developed which have significant advantages as compared to the conventional high pressure process. One such low pressure process is disclosed in commonly-assigned, copending U.S. applications Ser. No. 892,322, filed Mar. 31, 1978, now abandoned, and Ser. No. 12,720, filed Feb. 16, 1979, now U.S. Pat. No. 4,302,565, the disclosures of which are hereby incorporated herein by reference (a foreign-filed application corresponding thereto has been published as European Patent Publication No. 4647). Ethylene polymers made by such a low pressure process may be formed into film by known techniques and such film is extremely tough and is useful in packaging applications.
The above-identified copending applications disclose a low pressure, gas phase process for producing low density ethylene copolymers having a wide density range of about 0.91 to about 0.94 g/cc and a melt flow ratio of from about 22 to about 36 and which have a relatively low residual catalyst content and a relatively high bulk density. The process comprises copolymerizing ethylene with one or more C.sub.3 to C.sub.8 alpha-olefin hydrocarbons in the presence of a high activity magnesium-titanium complex catalyst prepared under specific activation conditions with an organo aluminum compound and impregnated in a porous inert carrier material. The copolymers (as applied to these polymers, the term "copolymers" as used herein is also meant to include polymers of ethylene with 2 or more comonomers) thus prepared are copolymers of predominantly (at least about 90 mole percent) ethylene and a minor portion (not more than 10 mole percent) of one or more C.sub.3 to C.sub.8 alpha-olefin hydrocarbons which should not contain any branching on any of their carbon atoms which is closer than the fourth carbon atom. Examples of such alpha-olefin hydrocarbons are propylene, butene-1, hexene-1, 4-methyl pentene-1 and octene-1.
The catalyst may be prepared by first preparing a precursor from a titanium compound (e.g., TiCl.sub.4), a magnesium compound (e.g., MgCl.sub.2) and an electron donor compound (e.g., tetrahydrofuran) by, for example, dissolving the titanium and magnesium compounds in the electron donor compound and isolating the precursor by crystallization. A porous inert carrier (such as silica) is then impregnated with the precursor such as by dissolving the precursor in the electron donor compound, admixing the support with the dissolved precursor followed by drying to remove the solvent. The resulting impregnated support may be activated by treatment with an activator compound (e.g. triethyl aluminum).
The polymerization process may be conducted by contacting the monomers, in the gas phase, such as in a fluidized bed, with the activated catalyst at a temperature of about 30.degree. to 105.degree. C. and a low pressure of up to about 1000 psi (e.g., from about 150 to 350 psi).
The resulting granular polymers may contain gaseous unpolymerized monomers including hydrocarbon monomers. These gaseous monomers should be removed from the granular resin for safety reasons, since there is a danger of explosion if the hydrocarbon monomer concentration becomes excessive in the presence of oxygen. In addition, proper disposal of the hydrocarbons is required in order to meet environmental standards concerning hydrocarbon emissions.
The prior art teaches techniques for removing volatile unpolymerized monomers from polymers of the corresponding monomers. For example, U.S. Pat. No. 4,197,399 discloses a process for removing residual vinyl chloride monomer present after polymerization of vinyl chloride polymers in the form of an aqueous dispersion. The process comprises heating the polymer at least to its glass transition temperature and stripping the polymer at the elevated temperature by an inert fluid such as air, nitrogen or steam.
U.S. Pat. No. 3,594,356 discloses a polymer recovery process for the removal of polymer from solvent. After an initial flashing operation and sizing the resulting solid polymer particles, the polymer is purged in two stages with an inert gas such as nitrogen. Any unreacted monomer may be removed by venting and/or flashing.
U.S. Pat. No. 3,450,183 discloses a process for the recovery of polyolefin solids from a solution containing same, which comprises flashing to obtain a solid concentrate and thereafter subjecting the chopped solids to a mixing action countercurrent to a stream of inert purge gas such as nitrogen. Any unreacted olefin can be removed by venting and/or flashing.